This invention relates to a permanent magnet rotor having permanent magnets provided at a rotor iron core and a brushless motor equipped with such a rotor.
As a rotor used for a motor, a permanent magnet rotor is known in which permanent magnets are provided at a plurality magnet attaching segments arranged at a rotor iron core. The permanent magnet rotor includes a type of having a convex (convex pole) between adjacent magnets (e.g. Patent Reference 1) and another type of having not the convex but a concave between the adjacent magnets (e.g. Patent Reference 2).
Both types of permanent-magnetic rotors and brushless motors equipped with these rotors have advantages and disadvantages, respectively.
The permanent magnet rotor of a type having the convex and brushless motor equipped with such a rotor have advantages that (1) reluctance torque is relatively large, and (2) the magnetic flux passes through the convex and hence easily passes through the yoke at a more inner side in a radial direction than the magnet so that the magnetic flux is difficult to be saturated, thus giving “senserless position detection” with a high accuracy, but also have disadvantages of more heat generation of the magnet and being fragile to demagnetization. The “senserless position detection” is to estimate the position of the rotor on the basis of the current flowing through each of windings in a multiple phase when a voltage for position detection is applied to the windings of a stator.
On the other hand, the permanent-magnetic rotor of a type having not the convex but the concave and the brushless motor equipped with such a rotor have advantages of less magnet heat generation and being tolerant to demagnetization, but disadvantages that (1) the reluctance torque is relatively small, and (2) because of absence of the convex, the magnetic flux from the stator is difficult to pass through the yoke at a more outer side in a radial direction than the magnet so that the magnetic flux is likely to be saturated, thus giving “senserless position detection” with a low accuracy, and (3) the thickness of the yoke on both sides in a circumferential direction of the magnet is decreased, thereby low strength against the centrifugal force during a high speed rotation.
Proposed are also a rotor in which an auxiliary pole portion is provided through a groove (concave) between permanent magnets adjacent to each other and a brushless motor equipped with such a rotor (e.g. Patent Reference 3). Patent Reference 3 describes that torque pulsation can be suppressed when the pitch of the slots in a stator, open angle of the magnetic pole of the permanent magnet in the rotor and the open angle of the auxiliary pole are set in their dimension so as to satisfy a prescribed relationship.                Patent Reference 1: JP-A-Hei5-76146        Patent Reference 2: JP-A-Hei10-285849        Patent Reference 3: JP-A-2002-305859        
Now, it is eagerly demanded to develop a rotor capable of satisfying all the requirements of an increase in reluctance torque, improvement in the sensorless position detection accuracy and suppression of magnet heat generation and a brushless motor equipped with such a rotor. The brushless motor disclosed in Patent Reference 3 cannot solve such a problem.